1. Field of the Invention
The present invention relates to a lift apparatus for artificial lift wells. More particularlly, the invention relates to an apparatus that monitors conditions in a well and makes automated adjustments based upon those conditions.
2. Background of the Related Art
In the recovery of oil from an oil well, it is often necessary to provide a means of artificial lift to lift the fluid upwards to the surface of the well. For example, when an oil-bearing formation has so little natural pressure that the oil is unable to reach the surface of the well after entering a wellbore through perforations formed in the wellbore casing. As the oil from the formation enters the wellbore, a column of fluid forms and the hydrostatic pressure of the fluid increases with the height of the column. When the hydrostatic pressure in the wellbore approaches the formation pressure of the well, i.e., the pressure acting upon production fluid to enter the wellbore, the oil may be prevented from entering the formation and its flow may be reversed. The resulting back flow may carry fluid and sand back into the formation and prevent future production into the wellbore. To avoid this problem, conventional wells utilize tubing coaxially disposed in the wellbore with a pump at a lower end thereof to pump wellbore fluid to the surface and reduce the column of fluid in the wellbore.
Artificial lift pumps include progressive cavity (PCP) pumps having a rotor and a stator constructed of dissimilar materials and with an interference fit therebetween. PCPs are operated from the surface of the well with a rod extending from a motor to the pump. The motor rotates the rod and that rotational force is transmitted to the pump. Effective and safe operation of artificial lift wells as those described above require an optimum amount of fluid be in the wellbore at all times. As stated above, the fluid column must not rise above a certain level or its weight and pressure will damage the formation and kill the well. Conversely, PCPs require fluid to operate and the pump can be damaged if the fluid level drops below the intake of the pump, leading to pump cavitation and pump failure due to friction between the moving parts.
To ensure that the optimum fluid level is maintained in the wellbore, conventional artificial lift wells utilize pressure sensors and automated controllers to monitor the fluid and pressure present in the wellbore. The pressure sensors are located at or near the bottom of the wellbore and the controller is typically located at the surface of the well. The controller is connected to the sensors as well as the PCP. By measuring the pressure in the annular area between the production tubing and the casing wall and by comparing that pressure to a known formation pressure for the well, the controller can operate a PCP in a manner that maintains the wellbore pressure at a safe level. Additionally, by knowing dimensional characteristics of the wellbore, the height of fluid can be calculated and the controller can also operate the pump in a manner that ensures an adequate about of fluid covers the PCP.
The conventional apparatus operates in the following manner: As the pressure in the wellbore approaches a predetermined value based upon the formation pressure of the well, the controller causes the pump speed to increase by increasing the speed of the motor. As a result, additional fluid is evacuated from the wellbore into the tubing and transported to the surface, thereby reducing the column of the fluid in the wellbore and also reducing the chances of damage to the well. If the hydrostatic pressure at the bottom of the wellbore becomes too low, the controller causes the speed of the pump to decrease to insure that the pump remains covered with fluid and has a source of fluid to pump.
There are problems associated with artificial lift apparatus like the one described above. One problem arises with the use of filters at the lower end of the production tubing string. The filters are necessary to eliminate formation sand and other particulate matter from the production fluid entering the tubing string. Filters typically include a perforated base pipe, fine woven material therearound and a protective shroud or outer cover. The filters are designed to be disposed on the tubing string below the pump in order to filter production fluid before it enters the pump. However, as the filters operate, they can become clogged and restrict the flow of fluid into the pump. The result of a clogged filter in the automated apparatus described above can be catastrophic due to the system""s inability to distinguish a clogged filter from some other wellbore condition needing an automated adjustment. For instance, with a clogged filter, the pump is unable to operate effectively and the fluid level in the wellbore increases. With this increase comes an increase in pressure and a signal from the controller to the pump motor to increase the speed of the pump. Rather than reduce the wellbore pressure, the pump continues to operate ineffectively due to the clogged filter and the pump motor begins to overheat as it provides an ever-increasing amount of power to the pump. Meanwhile, the fluid level in the wellbore continues to rise towards the formation pressure of the well. The combination of the increasing pump speed and the pump""s inability to pass fluid causes the pump to fail. After the pump fails, the wellbore is left to fill with oil and cause damage to the well.
Another problem associated with the forgoing conventional apparatus relates to the measurement of the annulus pressure. As fluid collects in the wellbore of an artificial lift well, air above the fluid column in the wellbore is compressed due to the fact that the upper end of the wellbore is typically sealed. As the air is compressed, the air pressure necessarily acts upon the fluid column therebelow and also upon the pressure sensor located at the bottom of the wellbore. The result is a pressure reading at the lower casing sensor that is a measure of not only fluid pressure but also of air pressure. While this combination pressure is useful in determining the overall pressure acting upon the formation, it is not an accurate measurement of the height of the fluid column in the wellbore. Therefore, depending upon the amount and pressurization of air in the upper part of the wellbore, an inaccurate calculation of fluid height results. Because the calculation of fluid height is critical in operating the well effectively and safely, this can be a serious problem.
There is a need therefore, for an artificial lift well that can be operated more effectively and more safely than conventional artificial lift wells. There is a further need for an apparatus to operate an artificial left well wherein a number of variables are monitored and controlled by a controller to ensure that the formation around the wellbore is not damaged and continues to produce. There is yet a further need for an artificial lift apparatus to ensure the safety of PCP pumps.
The present invention provides an artificial lift apparatus that monitors the conditions in and around a well and makes automated adjustments based upon those conditions. In one aspect, the invention includes a pump for disposal at a lower end of a tubing string in a cased wellbore. A pressure sensor in the wellbore adjacent the pump measures fluid pressure of fluid collecting in the wellbore. Another pressure sensor disposed in the upper end of the wellbore measures pressure created by compressed gas above the fluid column and a controller receives the information and calculates the true height of fluid in the wellbore. Another sensor disposed in the lower end the tubing string measures fluid pressure in the tubing string and transmits that information to the controller. The controller compares the signals for the sensors and makes adjustments based upon a relationship between the measurements and preprogrammed information about the wellbore and the formation pressure therearound. In another aspect the invention includes additional sensors for measuring the torque and speed of a motor operating a progressive cavity pump (xe2x80x9cPCPxe2x80x9d). In another aspect the invention includes a method for controlling an artificial lift well including measuring the wellbore pressure at an upper and lower end, measuring the tubing pressure at a lower end and comparing those values to each other and to preprogrammed values to operate the well in a dynamic fashion to ensure efficient operation and safety to the well components.